In most cases, the randomly poured bed or packing is employed in vertical vessels with the gas or vapor phase entering the packed bed from the bottom of the packed bed and rising upwardly. The liquid is administered to the top of the packed bed, usually by means of a liquid distributor, and through the force of gravity runs downwardly.
It is the function of the packing to provide a framework or open celled structure in the tower over the surface of which it is endeavored to pass the liquid in the form of a thin film, whereas the gas or vapor is rising upwardly through the interstices of this structure. With the liquid thus finely divided and offering much surface area, mass transfer is occurring between the gas or vapor and liquid phases, respectively.
With this mechanism being operative in the packed bed, it is readily conceived that the rate of mass transfer per unit volume of packed bed, hence the efficiency of the packed bed to bring about mass transfer, is for given systems and concentrations dependent, to an important extent, upon the surface area which the packing can provide.
Whereas it is not a particularly difficult problem to propose a shape of tower packing element, which, when poured into a vessel, will generate a packed bed of an unusually large surface area, this situation is by itself not sufficient to assure a high rate of mass transfer. Rather in order for a high rate of mass transfer to occur, one of the major requirements is that the surface area offered by the packing elements in the packed bed must be readily accessible to the descending liquid and rising gas. Thus, whereas in comparative beds of saddles, as are for instance described in U.S. Pat. No. 1796501 and U.S. Pat. No. 2639909, the total surface areas are essentially numerically equivalent for comparative sizes of packings, it has been found that mass transfer rates in beds of saddles constructed in accord with U.S. Pat. No. 2639909 are significantly higher, because due to the less symmetrical shape embodied in the saddle element of U.S. Pat. No. 2639909, there is no nesting of pieces. Hence the surface area offered to mass transfer is more readily accessible to liquid and gas contacting. The extent of irregularity of shape which is provided by a tower packing element is thus noted to have an important bearing on the resulting mass transfer efficiency.
Another important requirement for bringing about a high rate of mass transfer is that the packing elements composing the packed bed must comprise such features that are conducive to provide an efficient liquid distribution throughout the packed bed. Thus it has been found that in beds of slotted rings, such as are described in U.S. Pat. No. 3266787, the tongues contained in the interior of the slotted rings are largely ineffective as far as providing an improved internal liquid flow pattern through the packed bed is concerned. This is so because with the tongues all inside of the rings, the tongues are somewhat confined and not sufficiently exposed, therefore do not make any significant contact with adjoining packing pieces in the packed bed.
A significant improvement in internal liquid distribution and hence much better mass transfer was achieved by cutting the slotted rings lengthwise into halves and thus enabling the tongues of one packing element to make effective contact with the surface area and other tongues in neighboring packing elements in the packed bed. Through this simple measure, convenient flow paths have been provided by the tongues, along which the descending liquid can now readily pass from packing element to packing element, rather than fall through the packed bed at random. Thus a much improved state of wetting in the packed bed has been achieved, which has led to a dramatically improved mass transfer efficiency of the packed bed composed of longitudinal halves of slotted rings. The series of experiments demonstrating this important result is presented in a paper which the author has published in the October 1980 issue of VERFAHRENSTECHNIK.
Systematic research done by the author and described in U.S. Pat. No. 4203934 has shown that further important improvements of half-slotted rings will result by ascribing a definite geometry to the individual packing elements, as well as proposing definite designs and arrangements of the tongues.
Besides these two important requirements of a packing element, that is to provide a readily accessible maximum contact surface area, and to bring about an efficient internal liquid distribution, there are a number of other features that should be provided by an efficient packing element. Thus it is important that the packing elements will generate a stable bed configuration, so that the packing pieces will not continue to settle with time, and thus preclude an increase in the vapor or gas phase pressure drop with time.
Another important requirement is that the packing elements in the packed bed should be thoroughly interlocked with each other, so that the downwardly acting weight force of the packed bed is randomly distributed across the tower cross-section. In this way the weight force acting laterally on the tower wall will be minimized. This will also eliminate the danger of crushing of the lowermost packing layers in the bottom of a high packed bed. The ability of adjacent packing elements to interlock will permit the packing elements to be made of thinner gage material which is important as far as the economics of the structure is concerned.